A MEM switch is a switch operated by an electrostatic charge, thermal, piezoelectric or other actuation mechanisms and manufactured using micro-electromechanical fabrication techniques. A MEM switch may control electrical, mechanical, or optical signal flow. Conventional MEM switches are usually single pole, single throw (SPST) configurations having a rest state that is normally open. In a switch having an electrostatic actuator, application of an electrostatic charge to the control electrode (or opposite polarity electrostatic charges to a two-electrode configuration) will create an attractive electrostatic force (‘pull’) on the switch causing the switch to close. The switch opens by removal of the electrostatic charge on the control electrode(s), allowing the mechanical spring restoration force of the armature to open the switch. Actuator properties include the required make and break force, operating speed, lifetime, sealability, and chemical compatibility with the contact structure.
A micro-relay includes a MEM electronic switch structure mechanically operated by a separate MEM electronic actuation structure. There is only a mechanical interface between the switch portion and the actuator portion of a micro-relay. When the switch electronic circuit is not isolated from the actuation electronic circuit, the resultant device is usually referred to as a switch instead of a micro-relay. MEM devices are typically built using substrates compatible with integrated circuit fabrication, although the electronic switch structure disclosed herein does not require such a substrate for a successful implementation. MEM micro-relays are typically 100 micrometers on a side to a few millimeters on a side. The electronic switch substrate must have properties (dielectric losses, voltage, etc.) compatible with the desired switch performance and amenable to a mechanical interface with the actuator structure if fabricated separately.
MEM switches are constructed using gold or nickel (or other appropriate metals) as contact material for the device. Current fabrication technology tends to limit the type of contact metals that can be used. The contacts fabricated in a conventional manner tend to have lifetimes in the millions of cycles or less. One of the problems encountered is that microscale contacts on MEM devices tend to have very small regions of contact surface (typically 5 micrometers by 5 micrometers). The portion of the total contact surface that is able to carry electrical current is limited by the microscopic surface roughness and the difficulty in achieving planar alignment of the two surfaces making mechanical and electrical contact. Thus, most contacts are point contacts even on a surface that would seem to have hundreds or thousands of square micrometers of contact surface available. The high current densities in these small effective contact regions create microwelds and surface melting, which if uncontrolled results in impaired or failed contacts. Such metallic contacts tend to have short operational lifetimes, usually in the millions of cycles.
The state of the art in macro-scale relays/switches is well developed. There has been a considerable effort in developing long life contact metallurgy for the signal contacts. The signal contact life and the appropriate contact metallurgy tends to be rated by the application, such as “dry” signals (no significant current or voltage), inductive loads and high current loads.
It is known in the art, that electrical contacts using mercury (chemical symbol Hg) as an enhancement for switch contact conductivity yields longer contact life. It is also known that the Hg enhanced contacts are capable of operating at higher current than the same contact structure without mercury. Mercury wetted reed switches are an example. Other examples or mercury wetted switches are described in U.S. Pat. Nos. 5,686,875, 4,804,932, 4,652,710, 4,368,442, 4,085,392 and Japanese application 03118510 (Publication No. JP04345717A).
The use of mercury droplets in a miniature relay (a device which is much larger than a MEM relay) controlled by a high voltage electrostatic signal is taught in U.S. Pat. No. 5,912,606. U.S. Pat. No. 5,912,606 uses the electrostatic signal on a gate to attract liquid metal drawn from a first contact to liquid metal drawn from a second contact or to draw liquid metal from both contacts to a shorting conductor mounted on the gate in order to electrically connect the contacts.
A conventional vertically activated surface micromachined electrostatic MEM micro-relay 10 structure is shown in FIG. 1. The MEM micro-relay 10 includes a single substrate 30 on which is micromachined a cantilever support 34. A first signal contact 50, a second signal contact 54, and a first actuator control contact 60a are disposed on the same substrate 30. The contacts have external connections (not shown) in order to connect the micro-relay to external signals. One end of a cantilever 40 is disposed on cantilever support 34. Cantilever 40 includes a second actuator control contact 60b. A second end of the cantilever 40 includes a shorting bar 52. The two conductive actuator control contacts 60a and 60b control the actuation of the MEM micro-relay 10.
Without a control signal, the shorting bar 52 on the cantilever 40 is positioned above the substrate 30 by the support 34. With the cantilever 40 in this position, the first and second signal contacts 50 and 54 on the substrate 30 are not electronically connected. An electrostatic force created by a potential difference between the second actuator control contact 60b and the first actuator control contact 60a on substrate 30 control connection is used to pull the cantilever 40 down toward the substrate 30. The MEM micro-relay 10 uses the conductive shorting bar 52 to make a connection between the two signal contacts 50 and 54 attached to the same substrate 30 as the cantilever 40 and cantilever support 34. When pulled to the substrate 30, the shorting bar 52 touches the first and second signal contacts 50 and 54 and electrically connects them together. The cantilever 40 typically has an insulated section (not shown) separating the shorting bar 52 from the cantilever electrostatic actuator control contact 60b. Thus, the first and second signal contacts 50 and 54 are connected by the cantilever 40 shorting bar 52, which is operated by an isolated electrostatic force mechanism using the two actuator control contacts 60a and 60b surfaces. The contacts 50, 54 and the shorting bar 52 typically have short operational lifetimes due to the problems described above.
The micromachined electrostatic MEM micro-relay 10 is shown as a normally open (NO) switch contact structure. The open gap between the actuator control contact 60a and the cantilever beam 40 is usually a few microns ({fraction (1/1,000,000)} meter) wide. The gap between the shorting bar and the signal contacts is approximately the same dimension. When the switch closes, the cantilever beam 40 is closer to but not in direct electrical contact with actuator control contact 60a. 
If the signal contact metal is wettable with mercury, and the rest of the micro-relay is not wettable, then the mercury could be deposited on the signal metalization and allowed to flow into the active contact area under the cantilever by capillary action. The problem of mercury bridging at these close spacings must be addressed. When the mercury contacts are not contained, the contacts are subject to all the problems described in the above referenced patents including splashing and the need for liquid metal replenishment.
Mercury contacts represent a major challenge for the conventional MEM switch. The typical physical separation between the contacts on the substrate and the shorting bar is a few micrometers to a few tens of micrometers. Placing mercury on the contact surfaces during the fabrication of the micro-relay requires that the chemical process be compatible with mercury or other liquid metals. Mercury has limited or no compatibility with typical CMOS processes used to fabricate vertical structure micro-relays.
The close separation between the shorting bar and the contacts makes it difficult to insert mercury on the contacts after the micro-relay is fully operational. Applying a mercury wetting to the fully functional contact and shorting bar surfaces would be difficult, and the problem of mercury bridging at these close spacings must be overcome. All the problems known to apply to macro-scale liquid contacts will likely apply to the structure of MEM micro-relay 10. The addition of liquid contacts to this MEM micro-relay design thus requires the use of a different construction technique and different contact systems.
A vertical structure MEM relay using electrostatic actuators can be fabricated with multiple anchor points and both contact springs and release springs as an alternative to the cantilever described in FIG. 1. An example of a radio frequency (RF) relay having contact and release springs is described in Micro Machined Relay for High Frequency Application, Komura et al., OMRON Corporation 47th Annual International Relay Conference (Apr. 19-21, 1999) Newport Beach, Calif. Page 12-1, and Japanese Patent Abstract, Publication number 11-134998, publication date May 21, 1999.
FIG. 2 shows a conventional MEM switch with a lateral actuator. The micro-relay 10′ has a substrate 32 supporting a lateral actuator 70 connected to a shorting bar support 44. A first conductive control contact 60a′ is mounted in the housing substrate 32 and a second conductive control contact 60b′ is mounted in the substrate 32. A shorting bar 52′ is disposed on the shorting bar support 44. A first signal contact 50′ and a second signal contact 54′ are disposed on the same housing substrate 30. The shorting bar 52′ places signal contacts 50′ and 54′ into electrical contact when the mirco-relay 10′ is in a closed position.
Applying liquid contacts to this conventional micro-relay structure is also difficult for the reasons described above. The typical physical separation between the contacts on the substrate and the shorting bar is a few micrometers. This makes it difficult to insert liquid metal (e.g. mercury) on the contacts after the MEM switch is fabricated.
There is a need in the art for further improvements in MEM relays eliminating the shortcomings of the existing technology. What is needed is a long life, high current, and high voltage contact structure combined with a MEM actuator to form a direct current (DC) or RF micro-relay fabricated using micro-electromechanical (MEM) processes. In some applications there is a need to use liquid metal contacts which do not include mercury because of environmental considerations.